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C L I N I C A L C O R R E L A T E S

Eye Abnormalities

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The Optic Nerve

The optic cup is connected to the brain by theoptic stalk, which has a groove, the choroidfissure, on its ventral surface. In this groove

are the hyaloid vessels. The nerve fibers of the retina returning to the

brain lie among cells of the inner wall of thestalk.

During the seventh week, the choroid fissurecloses, and a narrow tunnel forms inside theoptic stalk.

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As a result of the continuously increasing number of nerve

fibers, the inner wall of the stalk grows, and the insideand outside walls of the stalk fuse.

Cells of the inner layer provide a network of neuroglia thatsupport the optic nerve fibers.

The optic stalk is thus transformed into the optic nerve.Its center contains a portion of the hyaloid artery, latercalled the central artery of the retina.

On the outside, a continuation of the choroid and sclera,the pia arachnoid and dura layer of the nerve,respectively, surround the optic nerve.

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Coloboma may occur if the choroid fissure fails to close.

Normally this fissure closes during the seventhweekof development

When it does not, a cleft persists.Although such acleft is usually in the iris onlycoloboma iridis it mayextend into the ciliary body, the retina, the choroid,and the optic nerve.

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a common eye abnormality frequently associatedwith other eye defects.

Colobomas (clefts) of the eyelids may also occur.Mutations in the PAX2 gene have been linked with

optic nerve colobomas and may play a role in theother types as well.

Renal defects also occur with mutations in PAX2 aspart of the renal coloboma syndrome

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The iridopupillary membrane may persist instead

of being resorbed during formation of the anteriorchamber.

In congenital cataracts the lens becomes opaqueduring intrauterine life.

Although this anomaly is usually genetically determined,many children ofmothers who have had German measles(rubella) between the fourth and seventh weeks ofpregnancy have cataracts.

If the mother is infected after the seventh week ofpregnancy, the lens escapes damage, but the child maybedeaf as a result of abnormalities of the cochlea.

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The hyaloid artery may persist to form a cord or

cyst. Normally the distal portion of this vessel degenerates,

leaving the proximal part to form the central artery of theretina.

In microphthalmia the eye is too small; the eyeballmay be only two-thirds of its normal volume.

Usually associated with other ocular abnormalities,microphthalmia frequently results from intrauterine

infections such as cytomegalovirus and toxoplasmosis.

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Anophthalmia is absence of the eye.

In some cases histological analysis reveals someocular tissue. The defect is usually accompanied

by severe cranial abnormalities.

Anophthalmia is sometimes a clinical

characteristic of Trisomy 13 (Patau syndrome)which is a Gross Chromosomal Abnormality.

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Congenital aphakia (absence of the lens) andaniridia (absence of the iris)

rare anomalies that are due to disturbances ininduction and formation of tissues responsible forformation of these structures.

Mutations in PAX6 result in aniridia and may also

contribute to anophthalmia andmicrophthalmia.

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Cyclopia (single eye) andsynophthalmia (fusion of theeyes) comprise a spectrum of defects in

which the eyes are partially orcompletely fused

The defects are due to a loss ofmidline tissue that may occur as

early as days 19 to 21 of gestation orat later stages when facialdevelopment is initiated.

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This loss results in underdevelopment of theforebrain and frontonasal prominence.

These defects are invariably associated withcranial defects, such as holoprosencephaly, inwhich the cerebral hemispheres are partiallyor completely merged into a single

telencephalic vesicle.

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